A load sensing valve is, essentially, a variable proportioning valve. Very common on trucks, or anything designed to operate with different loads. It attaches to the body or frame, and has a lever attached to the axle. As the springs compress under load, the LSV permits more rear braking. With a light load, it reduces rear braking to avoid rear brake lockup.

If it's not attached, brake performance will suffer. I don't know if you'll have no rear braking at all, but you certainly won't have as good brake performance as you would with it working. Should be a simple matter to reattach the arm, or to replace whatever is broken.

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1989 300 SEL that mostly works, but needs TLC